Under such circumstances, the pitching aerodynamic couple on the aircraft varies as a function of its aerodynamic incidence in such a manner that:
below a first incidence threshold said aerodynamic pitching couple decreases with increasing aerodynamic incidence and increases with decreasing aerodynamic incidence;
between said first incidence threshold and a second incidence threshold greater than said first threshold, said aerodynamic pitching couple increases with increasing aerodynamic incidence and decreases with decreasing aerodynamic incidence; and
above said second incidence threshold said aerodynamic pitching couple decreases with increasing aerodynamic incidence and increases with decreasing aerodynamic incidence.
Consequently, if the aerodynamic incidence of the aircraft accidentally exceeds said first threshold, the aircraft enters a domain of unstable flight in which any increase in incidence causes the aerodynamic pitching couple to increase which in turn increases the angle of incidence. As a result, the aircraft suddenly pitches nose up, and this may be followed by the aircraft suddenly pitching nose down when said second threshold is exceeded, thus producing great discomfort for the passengers of the aircraft.
An object of the present invention is to remedy this drawback for an aircraft having a horizontal plane of adjustable deflection and air brakes that tend to pitch the aircraft nose down when in the deployed position.